A box contains 4 white and 6 black balls. A random sample of size 4 is chosen. Let X denote the number of white balls in the sample. An additional ball is now selected from the remaining 6 balls in the box. Let Y equal 1 if this ball is white and 0 if it is black. Find: a. Var (Y|X = 0) b. Var (X Y = 1)